3.3.2. Time Delays
Given the above, one might naively expect Fe/O to be constant everywhere. In fact, it is not. Older stars in our galaxy show lower values of Fe/O (see Wheeler, Sneden, & Truran 1989). Why is this? Because stars have non-negligible lifetimes. As a result, a temporal variation in the ratio of two elements will be observed if the two elements are produced in stars with different lifetimes (Tinsley 1980). Thus, oxygen, which is produced primarily in high mass stars, and returned to the ISM in type II supernovae, will show an enhancement at early times, and Fe, which is produced primarily in the type I supernovae of intermediate and low mass stars, will show an enhancement at later times. This is just one example of a breakdown of the IRA.